1. Field of the Invention
The present invention relates to an inkjet printer head driving apparatus and a control method thereof, and more particularly, to an inkjet printer head driving apparatus and a control method to minimize high-frequency noise components which may occur as a heating element is driven to eject ink through nozzles formed on a printer head.
2. Description of the Related Art
In general, a printer prints images or information processed by an external device, such as a computer, on a recording medium, such as a sheet of paper. The printer is mainly classified into one of a wire dot type, a thermal transfer type, and an inkjet type.
An inkjet printer head has a plurality of nozzles ejecting ink to form the images on the recording medium. The inkjet printer head of a printing mechanism in an inkjet printer functions to complete a printing job as requested, and is driven by a head driving device to fulfill printing jobs by ejecting an appropriate amount of printing ink on the recording medium.
FIG. 1 is a block diagram explaining a head driving device 100 of a conventional inkjet printer.
As shown in FIG. 1, the head driving device 100 for the inkjet printer includes a logic control unit 110, a latch unit 120, a gate array 130, a level shift unit 140, and a switching unit 150.
The logic control unit 110 has a decoder 112 to receive an encoded data signal from a microcomputer (not shown) controlling overall operations of the inkjet printer, and decoding the encoded data signal, and a shift register 114 synchronized with a clock signal CLOCK of a clock terminal thereof to receive data decoded in the decoder 112 and output the data to the latch unit 120.
The latch unit 120 latches data decoded in the logic control unit 110 according to a latch signal LATCH.
The gate array 130 includes a plurality of AND gates connected to receive an output signal of the latch unit 120 and a strobe pulse signal STRB to determine a heating time of heating elements R.
As shown in FIG. 2, the level shift unit 140 includes a level converter 142 and a buffer 144.
The level converter 142 raises a potential level of data transmitted from the gate array 130. For example, if the potential level ranging from 0V to 5V is outputted from the gate array 130, the level converter 142 steps up the potential level to an optimum driving potential level for the switching unit 150 which drives the heating elements R.
The buffer 144 buffers a voltage outputted from the level converter 142, shapes a waveform of the output voltage, delays the output voltage of the level converter 142 for a predetermined time, and outputs a result, such as the delayed output voltage.
The switching unit 150 is connected to cause a power supply Vph to be supplied to turn on and off the heating elements R according to output signals of the level shift unit 140. The switching unit 150 includes a plurality of FETs 152 used as switching devices and coupled between the power supply Vph and ground GND.
That is, the potential levels outputted from the level shift unit 140 are transmitted to gates of the FETs 152 of the switching unit 150 to form current paths between drains and sources, thereby heating the heating elements R to eject ink from selected nozzles.
FIG. 3 is a circuit diagram schematically showing a part of a printer head 170 connected to a flexible printed circuit board (FPCB) 160 to explain how an LC resonant circuit formed on the FPCB and the printer head affects the printer head as the switching unit for driving the heating elements R is turned on.
Referring to FIG. 3, the FPCB 160 is a printed board having wirings formed thereon to transmit electric power and electric signals to the printer head 170. FPCB cables of the FPCB 160 are coupled with bonding pads 172 provided in the printer head 170 to electrically connect the printer head 170 with a printer system.
At this time, the FPCB 170 includes resistance components R1 and R2 and inductance components L1 and L2 on the FPCB cables, and internal power lines of the printer head 170, connected with the FPCB through the bonding pads, have resistance components R3 and R4.
Further, the printer head 170 is generally attached to one side of an ink cartridge, and has a plurality of nozzles (not shown) through which ink is ejected, the heating elements R heating the ink to eject the ink through the nozzles, and the bonding pads 172.
As shown in FIG. 3, between a head power terminal Vph and the ground GND are connected in series the inductance components L1 and L2 and the resistance components R1 and R2 of the FPCB cables, the heating element R formed on the printer head 170, the FET 152 driving the heating element R, and resistors R3 and R4. Further, a capacitor C2 is connected in parallel with the heating element R between head power terminal (power supply) Vph and the ground GND.
Here, the FET 152 switches on and off according to an output signal of the gate array 130. For example, the FET 152 is turned on if a high-level signal is outputted from the gate array 130, and is turned off if a low-level signal is outputted from the gate array 130.
If the FET 152 is turned on with an output signal of the level shift unit 140, electric current flows through the heating element R, which forms an LC resonant circuit of the inductance component L1 and the capacitor C2 between the head power terminal Vph and the ground GND. Accordingly, as shown in FIGS. 4A through 4D, waveforms, such as a voltage VPH of the head power terminal Vph and a current waveform IR of the heating element R, are oscillated when the heating element R is driven with voltages A and B, and the oscillations are attenuated due to resistances of the heating element R and the resistance components R1 to R4 of power lines so that the voltage and current waveforms VPH and IR return to their original states. At this time, a resonant frequency becomes
  1      2    ⁢    π    ⁢          LC      which is generated in the LC resonant circuit. That is, the resonant frequency is changed by an inductance component L and a capacitance component C. Accordingly, as the inductance component L and the capacitance component C between the head power terminal Vph and the ground GND become larger, oscillating periods of the waveforms become longer.
The inductance component L occurring from the inductance components L1 and L2 becomes larger as the FPCB cables are extended. Further, the capacitance component C becomes a total sum of parasitic capacitances occurring among the gate, source, and drain of the FET 152 times the total number of the FETs 152, and a value of the capacitance component C becomes larger as the FETs 152 is scaled up in each size or in number.
In the meantime, since the FETs 152 are in a high impedance state when the FETs 152 are turned off, an oscillation attenuation effect due to the resistance components R1 to R4 is ignored so that the oscillations continue longer than at the time the FETs 152 are turned on.
Further, the more driven the FETs 152 are at the same time, the higher an oscillation level of the head power terminal Vph becomes, which causes an electrical state of the printer head 170 to be unstable so that the printer head 170 may be broken down, and which causes high-frequency signals to be supplied to the power lines so that electromagnetic interference (EMI) characteristics may become worse.
Accordingly, in order to minimize the influence of the capacitance and inductance components, the rising/falling times of a gate signal of the FETs 152 have to be designed large enough in consideration of a capacitor charging/discharging time. However, since the FETs 152 are manufactured using a plurality of semiconductor processes, it is difficult to meet the above requirements.